The invention relates to the processes (preferably in a moving bed) for the production from hydrocarbons of aromatic compounds, in which a hydrocarbon feedstock that is treated by a hydrogen-rich gas is transformed. It pertains more specifically to the regenerative reforming or to the more specific production of BTX (butene, toluene, xylenes) with continuous regeneration of the catalyst.
It relates more particularly to the stage of reduction of the catalyst and optionally also to the first reactor in which the reactions for dehydrogenation of the naphthenes that are contained in the feedstock for the most part take place.
The catalyst generally comprises a substrate (for example formed by at least one refractory oxide; the substrate can also include one or more zeolites), at least one noble metal (preferably platinum), and preferably at least one promoter metal (for example tin or rhenium), at least one halogen and optionally one or more additional elements (such as alkalines, alkaline-earths, lanthanides, silicon, elements of group IV B, non-noble metals, elements of group III A, etc.). The catalysts of this type contain, for example, platinum and at least one other metal deposited on a chlorinated alumina substrate. In a general way, these catalysts are used for the conversion of naphthenic or paraffinic hydrocarbons that can be transformed by dehydrocyclization and/or dehydrogenation, in reforming or for the production of aromatic hydrocarbons (for example production of benzene, toluene, ortho-, meta- or paraxylenes). These hydrocarbons are obtained from the fractionation of crude petroleums by distillation or other transformation processes such as catalytic cracking or steam-cracking.
These catalysts are extensively described in the literature.
The chemical reactions that are involved in the reforming process are numerous. They are well known; for the reactions that are beneficial to the formation of aromatic compounds and to the improvement of the octane number, it is possible to cite the dehydrogenation of naphthenes, the isomerization of cyclopentanoic cycles, the isomerization of paraffins, and the dehydrocyclization of paraffins; and for the harmful reactions, it is possible to cite hydrogenolysis and hydrocracking of paraffins and naphthenes. These various reactions have very different speeds and are strongly endothermic for the dehydrogenation reactions, exothermic for the other reactions. This is why the reforming process takes place in several reactors that undergo more or less significant temperature drops.
Experience shows that the dehydrogenation reactions of naphthenes occur in the first reactor or reactors.
The processes for reforming or for production of aromatic compounds were carried out at 40 bar 30 years ago; at 15 bar 20 years ago, and today, it is common to see reforming reactors that operate at pressures of less than 10 bar, in particular between 3 and 8 bar.
This drop in hydrogen pressure, however, is accompanied by a faster deactivation of the catalyst by coking. The coke that consists of a high molecular weight and a primarily carbon and hydrogen base is deposited on the active sites of the catalyst. The H/C molar ratio of the coke that is formed varies from about 0.3 to 1.0. The carbon and hydrogen atoms form condensed polyaromatic structures whose percentage of crystalline organization is variable based on the nature of the catalyst and operating conditions of the reactors. Although the selectivity of transformation of the hydrocarbons into coke is very low, the contents of coke accumulated in the catalyst can be significant. Typically, for the fixed-bed units, these contents are between 2.0 and 20.0 to 25.5% by weight. For the circulating-bed units, these contents are spread from 3.0 to 10.0% by weight at the outlet of the last reactor. The coke is deposited for the most part in the last two reactors.
The coke deposition, faster at low pressure, also imposes a faster regeneration of the catalyst. The current regeneration cycles can drop up to 2-3 days.
Numerous patents deal with the processes for reforming or for production of aromatic compounds with continuous or sequential regeneration of the catalyst. The diagrams of processes use at least two reactors, in which a catalytic moving bed, through which passes a feedstock that consists of hydrocarbons and hydrogen, a feedstock that is reheated between each reactor, circulates from top to bottom.
Experience shows that the first reactor is the center of highly productive and fast hydrogen reactions.
Patent FR-2,657,087 of the applicant describes such a reforming process.